<p>Calculates the magnitude of a basin, deﬁned as the number of sources upriver with respect
to every point.</p>
<br>
<br>
<h3>Inputs</h3>
<ol>
<li>The !GridCoverage with the drainage directions, obtained with [Markoutlets]</li>
</ol>
<br>
<br>
<h3>Output</h3>
<ol>
<li>The !GridCoverage containing the matrix of the basin magnitude</li>
</ol>
<br>
<br>
<h3>Notes</h3>
<p> If the river net is a trifurcated tree (a node in which three channels enter and one exits),
then between number of springs and channels there exists a bijective correspondence hc = 2ns − 1
where <i>hc</i> is the number of channels and <i>ns</i> the number of sources; the magnitude is then also an
indicator of the contributing area.</p>
<br>
<br>
<h3>References</h3>
<ul>
<li>[Bibliography#9, 9] M.J. Woldenborg,1982</li>
<li>[Bibliography#8, 8] I. Rodriguez-Iturbe, 1997</li>
<li> [Bibliography#10, 10] J.S. Smart, 1976</li>
<li> [Bibliography#11, 11] J.S. Smart, 1976</li>
<li> [Bibliography#12, 12] R.L. Shreve, 1966</li>
<li> [Bibliography#13, 13] R.L. Shreve, 1967</li>
<li> [Bibliography#14, 14] W.R. James, 1969</li>
</ul>
